Generation X Factor has found its perfect comedy duo in Frisky and Mannish, a karaoke-cabaret act who belt out medleys of pop hits. The joke is that F&M are "pop educators", reconfiguring familiar songs for pedagogic (ie, comic) effect. But Laura Corcoran and Matthew Jones's hearts are as much in the singing as the send-up. They've got terrific voices, and it often seems that mickey-taking Mariah Carey and Florence Welch is just Corcoran's way of saying, "Whatever they can do, I can do better."

Nothing wrong with that, save that the show is sometimes emphatic to the point of being shrill. The audience-participation opening made me feel less frisky and mannish, more pestered and peevish – at least until its amusing nod to Minnie Riperton's most famous high note. Proceedings pep up thereafter. Corcoran's schoolmarm-in-a-corset Frisky (Joyce Grenfell given an Ann Summers makeover) gives us Whigfield's Saturday Night as a breathy torch song. Jones's Mannish throws unrestrained shapes in a house remix of Radiohead's Creep.

The music does not always serve the comedy: Jones undermines the parallels between Lily Allen and Noël Coward by performing his Coward ditties too fast. And, for all their talk of being educators, F&M neglect the uninitiated. If, like me, you are rusty on your Mark Morrison, the joke about how similar he sounds to Diana Vickers will register only weakly – and the duo rattle through snatches of tunes with undue haste. But there is always another pastiche gag along soon, many of them funny (a horror movie revamp of Total Eclipse of the Heart) or oddly touching (Chesney Hawkes's The One and Only as an existential breakdown). At such points, F&M make the familiar deliciously odd; elsewhere, they just make the rafters shake.